This approach was first translated in a clinical trial involving 45 adults with hematological malignancies who were not candidates for standard conditioning because of older age or serious co-morbidities. Using a single 200 cGy dose of TBI, sustained engraftment was achieved in 80% of cases and, remarkably, transplant related mortality was only 6.7% in this frail group of patients at 14 months. It is also notable that these transplants were performed primarily in the outpatient setting-the median length of hospitalization was 1 day. Low-dose TBI based conditioning has also been safely and effectively utilized for infants and children with severe combine immune deficiency and other severe immune deficiencies, undergoing related and unrelated donor transplantation. This clinical experience strongly suggests that if an effective low-dose TBI conditioning regimen can be developed for children with non-malignant diseases it could transform BMT from a costly, highly morbid, and sometimes life-taking procedure to a relatively inexpensive, safe and well-tolerated one.